Genetic counseling: First Trimester Screening
First Trimester Screening Overview *Screen low-risk population to identify those at higher risk to give birth to child with a chromosome abnormality **Normal result does not mean that there are no abnormalities **Abnormal result indicates that diagnostic testing by amniocentesis or CVS is warrented *Consists of ultrasound and blood test *Performed between 10th and 14th week of pregnancy Procedure *Biochemical screening **Blood draw done by finger stick onto filter paper **May be taken between 8 weeks, 4 days and 13 weeks, 3 days **Measures levels of substances found in the blood of all pregnant women ***FreeBeta Human chorionic gonadotropin (hCG) measurements ****Glycoprotein derived from the placenta ****Serum levels tend to be high in patients carrying a fetus affected with Down Syndrome (average = 1.9 MoM) ****Serum levels significantly reduced in patients carrying a fetus affected with Trisomy 18 (average = 0.18 MoM) ***Pregnancy Associated Plasma Protein A (PAPP-A) ****Produced by trophoblast of placenta ****Significantly reduced in blood of women carrying fetuses with Down Syndrome (average = 0.44 MoM) ****Significantly reduced in blood of women carrying fetuses with Trisomy 18 (average = 0.32 MoM) *Ultrasonagraphic findings **Measure nuchal translucency (NT) ***Amount of fluid that accumulates behind the neck of the baby ****Echo-free space between skin and soft tissue overlying cervical spine ****Present only during first trimester ***About 30-86% of fetuses with increased nuchal translucency have chromosomal abnormality ***Increased in fetuses with chromosome abnormalities, cardiac defects, and certain genetic syndromes ****Greater than 3.5 mm is considered out of normal range ****Increased NT is not diagnostic of chromosome abnormality ****Lymphedema seen in NT may be result of fetal cardiac abnormalities ***Usually resolves in second trimester ***No consensus on how to measure nuchal translucency resulting in conflicting results about effectiveness **Measured transbdominaly in 95% of cases and transvaginally in other 5% **Procedure takes between 15 and 30 minutes Detection Rate *Biochemical screening without NT **Identifies greater than 60% of Down Syndrome **Identifies 90% of Trisomy 18 **About 1 in 25 women with an increased risk based on screening will have baby with Down Syndrome *Biochemical screening with NT **Identifies about 90% of Down Syndrome **Identifies up to 97% of Trisomy 18 **False positive rate is 5% **About 1 in 17 women with an increased risk based on screening will have baby with Down Syndrome *Cannot screen for open neural tube defects so AFP testing still recommended in 2nd trimester *Cannot identify all birth defects Benefits and Limitations of Testing *Benefits **Helps reduce maternal anxiety at earlier gestational age **Allows patient to have CVS for diagnosis in 1st trimester **Allows for 1st trimester termination of affected fetus **Not an invasive procedure - no risk to mother or to fetus *Limitations **Currently not covered by insurance **Only screens for two out of possible thousands of genetic conditions **Screening will miss about 10% of Down Syndrome **Still requires invasive procedure for diagnosis **Not as sensitive if multiple pregnancies **Patient specific risks rely on accuracy of clinical information that is interpreted for results of test Research Study at Magee-Womens Hospital *Eligibility criteria: women 18-45 between 10 and 14 weeks *To determine if first trimester screening is an effective method to detect Down Syndrome and other chromosome abnormalities **If risk higher than 1 in 270, can continue routine prenatal care, undergo CVS at 10-13 weeks, or have amniocentesis after 15 weeks **Women who choose to participate must provide information on pregnancy outcome *Responsible for all costs, including blood sample, ultrasound, and genetic counseling References *"ACOG Committee Opinion: First Trimester Screening for Fetal Anomalies with Nuchal Translucency." American College of Obstetrics and Gynecology. http://www.medem.com *"NIH News Release: First Trimester Screening for Down Syndrome Possible, NICHD-Funded Study Finds." National Institutes of Health. http;//www.nih.gov *"Screening for Down Syndrome, Trisomy 18 Syndrome, and Open Neural Tube Defects in the First Trimester." GeneCare Medical Genetics Center". http://www.genecare.com *Spencer K. "Accuracy of Down Syndrome Risks Produced in a First-Trimester Screening Programme Incorporating Fetal Nuchal Translucency Thickness and Maternal Serum Biochemistry." Prenatal Diagnosis (2002) 22;244-246. Notes The information in this outline was last updated in June 2002. This material has been imported fom the wikibook "Genetic counseling"[ http://en.wikibooks.org/wiki/Genetic_counseling] under the GNU Free Documentation License.